Optical Coherence Tomography (OCT) and Optical Low Coherence Reflectometry (OLCR) are interferometry-based techniques that have been successfully used in non-invasive and non-destructive analysis and imaging of structures in turbid media, especially in biological tissues. OLCR is a one-dimensional optical ranging technique where the amplitude and longitudinal delay of broadband light scattered from a sample is resolved using a low-coherence interferometer. OCT constructs a two-dimensional transverse image of the sample from a series of one-dimensional scans; it is a non-invasive, non-destructive and non-contact imaging method that typically uses a low coherence interferometer to extract depth-resolved sample information, and a scanning system to build a 2D image. Recently, Fourier-domain OCT techniques such as swept-source OCT (SS-OCT) that utilizes a narrow-line swept-wavelength laser source have also been disclosed. Both OCT and OLCR techniques allow the localization of reflecting sites within a transparent or semi-transparent sample with a micrometer spatial resolution.
In both of these interferometric techniques, broadband or swept-frequency light traveling a reference path is mixed on the surface of a single or multiple detectors with light returning from or traversing a sample. With a broad-band light source, a variable delay line in the reference arm is used to select a small range of depth, conventionally referred to as the “coherence gate”, within the sample wherefrom the reflected or scattered signal results in interferometric fringes that can be detected and processed. The position of the coherence gate is defined and controlled by matching the optical path in two interferometer arms using the variable delay line.
Particular OCT implementations may take the form of a time-domain OCT or frequency domain OCT. Time-domain OCT is based on heterodyne interferometry, wherein light from the sample is combined with frequency shifted reference light, with the frequency shift resulting either from passing through an optical modulator located in the reference arm, or from the Doppler effect when the reference light is reflected from a moving reference mirror. Mixing of the sample and reference light in a square-law detector results in an electrical signal having DC and AC frequency components. The AC frequency component, which is caused by the interference of the sample light with the time-delayed reference light, is processed to extract sample information.
In the Fourier domain OCT the reference mirror position is fixed during the measurement, and the OCT setup is based on homodyne interferometry. The complete interferometric signal consists of DC components arising from non-mixing light from each of the arms, and interferometric components arising from mixed light.
The central part in both homodyne and heterodyne OCT systems is an interferometer, typically of a Michelson or a Mach-Zehnder type, illuminated for example by a low coherence light source. FIG. 1 illustrates a prior-art Mach-Zehnder-based OCT system 100 that is disclosed in U.S. Pat. No. 6,657,727 issued to Izatt et al, which is incorporated herein by reference; this interferometer can be implemented using inexpensive semiconductor light sources, e.g. wide-band LEDs, commercially available detectors, and flexible single-mode optical fibers suitable for remote imaging through minimally invasive diagnostic instruments.
In the OCT system 100, a Mach-Zehnder interferometer formed using two 2×2 couplers 102 and 104 is illuminated by a broadband light source 106; a sample 114 under examination is placed in a sample arm 108. A reference arm 110 includes a reflective delay line formed using a movable mirror 118, which is inserted into the reference arm 110 through a circulator 116. The sample arm 108 includes another circulator 112 which serves to illuminate the sample 114 with light coupled into the sample arm 108 by the coupler 102, and to direct light reflected from the sample 114 into the output coupler 104, wherein it is combined with light from the reference arm 110 and passed via its two output ports onto a balanced receiver 120, which includes two photo detectors D1 and D2 with differentially connected outputs. Due to the limited coherence length of the source, typically 10-15 microns, light transmitted through the reference arm 110 and light backscattered by internal sample reflections interferes constructively or destructively only when the interferometer arm optical path lengths are matched to within the source coherence length. Scanning the reference arm 110 length through a position corresponding to the depth of a reflecting site within the sample generates a localized interference pattern, which is recorded as a localized modulation of the detector current as a function of the reference arm position. The balanced receiver 120 current generated by a sample containing multiple reflecting sites distributed along its depth, such as biological tissue, contains the sum of multiple, overlapping copies of this interference pattern. A map of tissue reflectivity versus depth, which is conventionally referred to as an A-Scan, is obtained by scanning the reference mirror 118 at constant velocity, while recording the envelope of the detector current, e.g. by demodulating the detector current at the resulting Doppler frequency. Cross-sectional images of the sample backscatter, typically referred to as “B-Scans”, may be acquired by obtaining sequential A-scans while scanning the probe beam across the tissue surface using a lateral scanning optic device. The resulting two-dimensional datasets are plotted as gray-scale or false-color images.
When the optical path difference for light raveling in the reference and sample arms of a low-coherence interferometer is zero, the OCT receiver 120 generates a signal which has an interferometric component Is max. As the optical path difference increases far beyond the coherence length of the used source, the receiver generates a noise signal which is conventionally characterized by the standard deviation σi of the receiver photocurrent. The signal to noise ratio (SNR) Is max2/σi2 is an important characteristic of an OCT interferometer, which defines the image contrast for the sample.
To obtain a high-contrast image from a turbid medium, such as a biological sample, an imaging system should have a high SNR. A significant advantage of using a low-coherence interferometer, such as the interferometer 100, for signal detection is that the mixing of the reference light with the light scattered from the sample at the square-low detector provides a dramatic increase in the signal to noise ratio (SNR) and the dynamic range, as compared to direct detection of the scattered light. Indeed, since the interferometric component of the detector current is proportional to the product of the electric field amplitudes returning from each arm, the detected envelope signal is proportional to the square root of the sample reflectivity; as the result, very small reflections in the sample on the order of 10−11 of the incident power can be detected in A-scans recorded in a fraction of a second.
The Mach-Zehnder based interferometric system 100 shown in FIG. 1 provides additional SNR gain by optimizing the power splitting ratio α1 of the optical coupler 102, and by using a balanced differential receiver 120 instead of a single photo detector. As described in U.S. Pat. No. 6,657,72, the dual-balanced detection has two advantages: first, the light intensity incident on detectors D1 and D2 as a function of reference arm delay is 180° out of phase due to a known property of a 2×2 coupler, so that the envelope of difference signal between the two detector currents is equal to twice the amplitude of the AC component of the photocurrent of each detector; and, secondly, any excess noise originated from the low-coherence source 106 will be common to both detectors and therefore will be eliminated by the difference operation.
The interferometric component of the receiver 120 signal depends sinusoidaly on the optical path length difference between the arms of the interferometer, and also on any additional phase delay between the reference and sample arm fields. When this phase term is zero, the interferometric signal varies as a cosine of the optical path length difference between the arms, and when the phase term is 90 degrees, the interferometric signal varies as a sine of the path length difference. The zero and 90 degree phase delayed versions of the interferometric signal are commonly referred to as the real and imaginary components, or zero and 90 degree quadrature components, of a complex interferometric signal I.
A limitation of the interferometric system 100, as well as many other prior-art interferometric systems used in Fourier domain OCT imaging, is that it provides only one of two quadrature components of the interferometric signal resulting from the mixing the sample and reference light, or, equivalently, only a real part of a complex interferometric signal, so that information carried by the imaginary part of the signal is lost. One drawback resulting from this limitation of conventional single-channel OCT systems is the appearance of the co-called complex conjugate artifact, due to which positive and negative distances in an OCT scan are not resolved, so that only half of the potentially available imaging depth can be realized.
Another drawback of the prior-art single-channel OCT systems is that the detected interferometric signal typically depends on both the refractive and absorptive properties of the imaged sample, and it becomes difficult to separate them and obtain refractive and absorptive properties of the imaged sample individually. Prior-art attempts at such separation have been based on so-called Kramers-Kronig (KK) relations, which connect frequency dependencies of real and imaginary parts of a complex refractive index in one integral relationship. This, however, requires first acquiring, for example, the absorption coefficient of a sample in a wide spectral range, before the refraction coefficient of the sample at a given frequency can be computed. This approach has considerable drawbacks, since it requires expensive widely-tunable sources of light for performing OCT measurements over a wide wavelength range, for example from 200 nm to 1000 nm, with a relatively small frequency step, and the results of KK-computations are very sensitive to the accuracy of the initial absorption spectra measurements. An example of such approach is disclosed, for example, in an article entitled “Oxygen Saturation-Dependent Absorption and Scattering of Blood”, by Dirk J. Faber et al, published in Phys. Rev. Letters, V. 93, No 2, 9 Jul. 2004.
U.S. Pat. No. 7,019,838 to Izatt et al, which is incorporated herein by reference, discloses an OCT system that is enhanced for simultaneous acquisition of both quadrature components of the complex interferometric signal; the system, which is shown in FIG. 2 corresponding to FIG. 9 of the '838 patent, is based on a 3×3 optical coupler 40 used as a beamsplitter in a Michelson interferometer configuration, and uses specific optical properties of optical couplers having more than 2 optical ports. In particular, light entering a 3×3 optical coupler via two of its input ports will exit the coupler with a phase shift of 120°, or 2π/3, between light intensity outputs from any two of the three ports, provided that the coupler evenly splits the light between the output ports. In the shown configuration, the broadband light from the source 20 is coupled into one input port of the coupler 40 via a circulator 21. The coupler 40 splits the light along fibers F1 and F2 which respectively lead to the reference and sample arms. The reference arm terminates with a reflector providing a variable optical delay; the sample shown as a fly serves as the reflection source of the other arm. Light beams reflected from the reference and sample arms are re-combined in coupler 40. Two portions of this re-combined light are then detected by detectors D2 and D3, and a third portion of this light is input into coupler 2, where it is detected by detector D1. Each channel of the interferometer, i.e. each of the detectors D1-D3, measures an output signal with a phase shift of 120° relative to the other two output channels. Using the output signals of any two channels, the real and imaginary components of the complex ac interferometric signal can be obtained.
One drawback of the OCT system shown in FIG. 2 is that it lacks balanced detectors that enable to eliminate common mode noise in system 100 shown in FIG. 1, and the description in '838 patent does not provide any clear way how the differential detection can be realized in the system described therein. Another drawback of this system is that it is based on a Michelson configuration with a single coupler 40 functioning both as a beam splitter and a beam combiner, which does not allow the same flexibility as a Mach-Zehnder configuration in optimizing optical power splitting between the reference and sample arms. In a further disadvantage, about one third of the optical power of the source 20, which is directed along fiber F3, does not contribute in the interferometric signal. Also, the '838 patent does not provide a way to obtain refractive and absorptive properties of the sample using the disclosed system.
An object of the present invention is to provide an interferometric system that employs differential detection to obtain both quadrature components of a complex interferometric signal with enhanced SNR.
Another object of the present invention is to provide an interferometric system and method to generate refraction and absorption properties of a sample simultaneously by utilizing both quadrature components of a complex interferometric signal that is obtained using differential detection.